Ye olde bird nerds,
I was sketching out some plans to rebuild my practically-failed
Eggbeater antenna project for
70cm, as I am tired of cranking the rotator knob every pass and want to
try out a good omni satellite antenna, which I
can hang from the top limb of a tree 30 feet above my shack window (best
option on my city lot).
My plan is to build on the K5OE Eggbeater II antenna. One thing that's
always bugged me about these antennas
is that most are implemented using a 93 ohm coaxial section to phase the
loops. This is hard stuff to find, and though
I know some hams say, "anything below 1.5 SWR is OK", I'm a
perfectionist and want to try to polish it up.
The current antenna has a phasing section made out of two runs of RG-8X
coax with the shields shorted and insulated, centers fed as a balanced
100 ohm (theoretical) matching section. It was difficult to build and I
doubt its integrity because it's coiled up so tightly;
I don't think that the foam core coax retains its impedance well when
folded up in 1" PVC.
I was thinking about the feeding issues, then I recalled that one
popular CP yagi antenna design is to space the horizontal and vertical
elements 1/4 wavelength along the axis, and then feed them in phase. It
then came to me that it may be possible to build an eggbeater with the
two elements separated vertically by 1/4 free space wavelength, fed with
simple half-wave coax sections to replicate the two loops' 100 ohm
impedance into a simple coax tee. Polarity switching should be possible
with a coax relay and a 1/2 wavelength delay line. Radials would be
placed 1/4 wave apart as well, same spacing as the loops.
The advantages of this to me seem numerous. First of all, the free space
quarter wavelength spacing is easier to accurately implement than a 1/4
wave phasing section using coaxial cable at 70cm, and the
power/currents/etc split to the two antennas are more balanced as long
as you build the 1/2 wave sections to equal lengths, leading to improved
circularity skyward. You don't need to find rare coax for the phasing
section, and instead of cramming several folded up or coiled runs of
coax inside a narrow (for aesthetics) PVC tube, you only need to run two
half-wavelength (or any multiple of half-wavelength) runs further down
the mast, where the tee or relay could be ziptied.
So, that's my plan. If it makes sense, I'm going to give it a shot with
1/4" copper fridge tubing for the loops and some sort of sturdy wire
(coathangers? heh) for the radials, using LMR240 coax for the two
halfwave sections. If anybody has any input or comments, I'd love to
hear them.
(P.S. If somebody's done this and I'm reinventing the wheel, please let
me know. :) )
Matt
KC4YLV